Energy Encryption for Wireless Power Transfer - IEEE Xplore

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 9, SEPTEMBER 2015

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Energy Encryption for Wireless Power Transfer Zhen Zhang, Member, IEEE, K. T. Chau, Fellow, IEEE, Chun Qiu, and Chunhua Liu, Senior Member, IEEE

Abstract—This paper presents a novel energy encryption strategy for wireless power transfer (WPT) systems, which can effectively improve the security performance of wirelessly transferred energy. In a WPT system, energy is expected to transfer to specific receptors as well as to switch off other unauthorized energy transmission channels, so the security of energy transmission is an important issue. In the proposed secure WPT system, the energy is encrypted by chaotically regulating the frequency of the power source. Then, the authorized receptor can receive the energy by simultaneously adjusting the circuit to decrypt the encrypted energy based on the security key obtained from the power supply, while the unauthorized receptor cannot receive the energy without knowledge of the security key. Hence, a secure energy transmission channel is established to effectively prevent unauthorized receptors from stealing the energy. In this paper, both simulation and experimental results are provided to verify the validity of the proposed encrypted WPT system. Index Terms—Contactless charging, energy encryption, magnetic resonant coupling (MRC), security, wireless power transmission (WPT).

I. INTRODUCTION S one of the most epoch-making technologies, the wireless power transfer (WPT) system is increasingly attracting attentions in various application fields [1], [2], such as charging portable electronic devices, implanted medical devices, integrated circuits, and solar-powered satellites. In addition, many studies show that this innovation is particularly suitable for electric vehicles (EVs) [3], such as battery charging for normal vehicular operation [4], and energy exchange or energy arbitrage for advanced vehicle-to-grid (V2G) operation [5]. Thus, the WPT technology is not only changing our traditional usage pattern of the energy, but also showing significant meanings on the pervasive application of sustainable energies. According to the transmission distance, the WPT system can be classified into two groups: the near-field transmission and far-field transmission. The near-field power transmission utilizes electromagnetic field couplings, such as the inductive, capacitive, and magnetic resonant coupling (MRC) mechanisms. For example, Wang et al. presented a transient load detection model, which can be effectively used to detect load conditions for inductive power transfer (IPT) systems [6]. Then, Lee et al. proposed a dynamic

A

Manuscript received May 28, 2014; revised September 17, 2014 and July 10, 2014; accepted October 13, 2014. Date of publication October 17, 2014; date of current version April 15, 2015. This work was supported by the Grant (Project No. SPF 201109176034) from The University of Hong Kong, Hong Kong, China. Recommended for publication by Associate Editor M. A. E. Andersen. The authors are with the Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPEL.2014.2363686

IPT system to automatically increase the field strength for the improvement of the energy transfer efficiency and emission [7]. In 2011, an IPT system was proposed with a narrow rail width for charging roadway-powered online EVs, which can provide energy across the entire width of the roadway surface [8]. For plug-in EVs and V2G services, a bidirectional inductive power interface was developed, which can facilitate simultaneous and controlled charging or discharging of multiple EVs [9]. In 2014, the mutual coupling effect between planar inductors was modeled for IPT systems [10]. In addition, Mayordomo et al. published an overview article, which discussed about the advances and challenges of the IPT system in detail [11]. Besides, a capacitively coupled contactless power transfer (CCPT) system was proposed and implemented, which can penetrate through metallic materials and, thus, effectively avoid the eddy current in IPT systems [12]. In 2013, Liang et al. made a technical overview on the mechanism of CCPT systems [13]. The MRC-based WPT system was experimentally demonstrated in 2007 [14], which significantly promoted the development of WPT techniques. In 2009, a MRC-based WPT system was developed to wirelessly transfer the energy to multiple small receivers and EVs [15]. In 2011, Cheon et al. proposed circuit-model-based analysis for the MRC mechanism [16]. Meanwhile, Imura and Hori maximized the power transfer efficiency and air-gap length using the equivalent circuit and Neumann formula [17]. In 2012, the wireless domino-resonator system was proposed by the adopting noncoaxial axes and circular structures [18]. Also, it was analyzed from the perspective of magnetic coupling effect of nonadjacent resonant coils [19]. In 2013, an automated impedance matching system was also proposed to improve the power transfer efficiency by matching the resonant frequency of the resonator pair with that of the power source [20]. Besides, the double-layer printed spiral coil was also analyzed for MRC-based WPT systems [21]. The far-field power transmission deals with long-range power transfer, such as for the low-power sensor network, space, and military applications. Typically, the power can be wirelessly transferred via the microwave. In 2012, it presented a novel rectenna architecture for the microwave WPT technique, which ensures the best possible energy conversion efficiency over a very wide range of input power levels [22]. In addition, the laser can be also utilized for such long-range power transfer. Summerer and Purcell elaborated the concept of the laser-based WPT technique [23]. As aforementioned, many researchers have made fruitful achievements on the working principle, circuit topology, and transfer efficiency of various WPT systems. However, the study on the security issue is nearly unexplored; therefore, limiting the application of the WPT technique [24]. For instance, the multireceptor WPT system such as the EV charging system [25], [26] desires a high security of the transferred energy. Therefore, the

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Fig. 1.


Usage scenarios of WPT systems for EVs.

purpose of this paper is to propose and implement a novel energy encryption strategy which can enable secure power transfer for various WPT systems. In recent years, the chaos theory has been successfully applied to various applications, such as the pulse width modulation [27], industrial mixing [28], and electric compaction [29]. In particular, the chaotic encryption has been widely used for secure telecommunication [30]. In this paper, the proposed energy encryption scheme will newly employ the chaos theory, which is used to wirelessly transfer the energy to authorized receptors, as well as prevent unauthorized receptors from stealing the energy without knowledge of the security key. The rest of this paper is organized as follows. Section II will discuss about the topology and the working principle of the MRC-based WPT system. Section III will present the methodology of the proposed energy encryption scheme in detail. In Section IV, both the simulation and experimental results will be given to illustrate the validity of the proposed scheme. Finally, conclusions will be drawn in Section V. II. WIRELESS POWER TRANSMISSION SYSTEM As a promising energy transmission technique, the WPT system is showing great application prospects in various fields. Particularly, the wireless-charging technology has a great significance for EVs, which can effectively extend the driving range without requiring additional batteries. Fig. 1 depicts three possible usage scenarios, including charging and discharging in the parking lot, online energizing in the roadway-powered system, and the intelligent energy management in the smart home. As aforementioned, WPT technologies can be realized by three major mechanisms, namely the electromagnetic induction, the magnetic resonance, and the microwave. As the main approach for the far-field power transmission, the microwave has been studied for several decades. Although the energy can be wirelessly transferred by many meters away, the efficiency is relatively low. Besides, this method needs a high-working frequency, for instance, 900 MHz, 2.4, and 5.8 GHz [31], which is not suitable for the high-power energy transmission. Among the near-field WPT technologies, the magnetic induction method can realize energy-efficient transmission of 70–90%. However,

it is mainly utilized in a short-distance power transfer. For a relatively long-distance power transmission in the near field, the MRC method is the preferred solution, which has been identified to be most promising in foreseeable future. Thus, this paper adopts the MRC-based WPT system to realize energy encryption. As depicted in Fig. 2, the transmission part of the MRCbased WPT system comprises of three basic units: the primary, resonant, and secondary, where rp , rr , and rs are the internal resistances of the primary, resonant, and secondary coils, respectively, RL is the load resistance, Cp , Cr , and Cs are the capacitances of the primary, resonant, and secondary units, respectively, and Lp , Lr , and Ls are the inductances of the primary, resonant, and secondary units, respectively. Since the corresponding cross-coupling effect is very weak, it can be neglected in the analysis. In the primary unit, the impedance Zp can be calculated as Zp = jωLp +

1 + rp jωCp

(1)

where ω is the switching frequency of the power supply. Meanwhile, the input voltage Vin can be expressed as Vin = Vp + I p rp +

1 Ip jωCp

(2)

where Ip is the current of the primary unit, which is normally maintained at a constant value. The primary coil voltage Vp can be written as Vp = jωLp Ip − jωLpr Ir

(3)

where Lpr is the mutual impedance between the primary and resonant coils, and Ir is the current of the resonant unit. In the resonant unit, the impedance Zr is given by Zr = jωLr +

1 + rr jωCr

(4)

Thus, the current Ir can be calculated based on the induced potential by the primary coil and the cross-coupling effect with the secondary coil, which is given by Ir =

jωLpr Ip − jωLr s Is Zr

(5)

where Is denotes the current of the secondary unit, and Lr s denotes the mutual impedance between the resonant and secondary coils. In the secondary unit, the impedance Zs can be calculated as Zs = jωLs +

1 + rs . jωCs

(6)

Similarly, from the induced potential by the resonant unit, Is can be obtained as jωLr s Ir Is = . (7) Zs + RL In addition, the apparent power of the transferred energy Strans relies on the reflected impedances to the primary unit, which is given by Strans = Ip2 Zr p

(8)

ZHANG et al.: ENERGY ENCRYPTION FOR WIRELESS POWER TRANSFER

Fig. 2.

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Basic topology of MRC-based WPT systems.

where Zr p is the reflected impedance from the resonant unit to the primary unit. It can be obtained as 2

Zr p =

L2pr (Zs

ω + RL ) . ω 2 L2r s − (Zs + RL )Zr

(9)

In order to offer the maximum power to the load, the power factor of the transferred energy should be maintained at the maximum value, namely reducing or even eliminating the reactive power Qtrans in the transmission channel. By substituting (4) and (6) into (9), it can be calculated as Qtrans = Im(Zr p )Ip2 ω 2 L2pr Ip2 (ω 2 L2r s − AC + BD)2 + (AD + BC)2 2 · (A + B 2 )D + ω 2 L2r s B =

where

(10)

⎧ A = (rs + RL ) ⎪ ⎪ ⎪ ⎪ ⎨ B = ωLs − 1 ωCs . C = rr ⎪ ⎪ ⎪ ⎪ ⎩ D = ωL − 1 r

ωCr

Consequently, it shows that the power factor of the transferred energy can achieve the maximum value by ensuring the equations as ⎧ 1 ⎪ =0 ⎨ ωLs − ωCs . (11) ⎪ ⎩ ωLr − 1 = 0 ωCr Besides, the primary circuit is also expected to work at the resonant frequency to eliminate the unnecessary reactive power. Hence, it should also ensure the equation as ωLp −

1 = 0. ωCp

(12)

Therefore, the maximum power of the transferred energy can be achieved when the primary, resonant and secondary units have an identical resonant frequency ω0 which is given by ω0 =

1 1 1 =√ =√ . Lr Cr Ls Cs Lp Cp

(13)

III. ENERGY ENCRYPTION SCHEME In WPT systems, the power flow performances including the transferred power, efficiency, and distance can be controlled by regulating the working frequency. In particular, the transferred power is significantly affected by the switching frequency [32]–[34]. In other words, the power can be efficiently transferred to the receptor with the optimal switching frequency, while the transferred power can be suppressed at an extremely low level when the frequency deviates from the optimal value. Accordingly, this frequency sensitivity causes technical difficulty in tracing the maximum power point for WPT systems. The key of energy encryption is to positively utilize the frequency sensitivity. Namely, the optimal frequency is purposely adjusted according to the predefined regulation by the power supply unit. The rule of frequency variation is confidential, and unpredictable for all potential energy receptors. In such way, the wirelessly-transferred energy is split by the specified time slot and meanwhile packed with various frequencies. Accordingly, the rule of the frequency variation is the key to open the energy package. In other words, when the frequency is adjusted to vary with a predefined secure sequence, the unauthorized receptors cannot retrieve the transferred energy without knowledge of the sequence of frequency regulation, while the authorized receptors can receive the energy by adjusting the receptor circuit based on the acquired security key. Thus, the proposed energy encryption scheme essentially utilizes the anticontrol of frequency regulation for WPT systems. Due to the unique characteristic of random-like behavior within an adjustable bounded domain, the chaotic sequence is utilized as the security key to encrypt the transferred energy. In this paper, the Logistic map is utilized to generate the 1-d discrete-time chaotic series as given by [35] ξi+1 = Aξi (1 − ξi ), A ∈ [0, 4]

(14)

where ξi denotes the sequence, and A denotes the bifurcation parameter. Fig. 3(a) depicts the 3-D bifurcation diagram where the phase portrait of ξi and ξi+1 exhibits various topological structures along with the increase of A. Specifically speaking, the ξi behaves as a constant value for A ∈ [0, 1), a period-1 oscillation for A ∈ [1, 3), a period-n oscillation for A ∈ [3, 3.57), and a chaotic oscillation for A ∈ [3.57, 4]. As depicted in Fig. 3(b), the largest Lyapunov exponent diagram also mathematically illustrates the chaotic behaviors. It shows that the

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Fig. 4.

Fig. 3. Logistic map. (a) 3-D bifurcation diagram. (b) Largest Lyapunov exponent.

largest Lyapunov exponent becomes positive whenA > 3.57, which quantitatively confirms that the infinitesimally close trajectories in the phase space exponentially diverge as time goes by, namely the chaotic behavior occurs. Consequently, A = 3.9 is selected to generate the random-like-but-bounded security key ξi ∈ (0, 1)for the proposed energy encryption scheme. Accordingly, the encrypted switching frequency can be obtained as ω = δi ω0

(15)

where δi is the chaotic security key, which can be expressed as δi = a + (b − a)ξi , 0 < a < b.

(16)

Then, the switching frequency can be regulated to chaotically vary within a regulating range of (b−a) ω 0 . In general, the regulating range can be arbitrarily chosen according to the power level and transmission distance. With the chaotic security key, the primary, resonant, and secondary capacitors can be accordingly regulated to maximize the transferred power, which are respectively expressed as Cp =

1 1 · 2 2 δi ω0 Lp

(17)

Cr =

1 1 · δi2 ω02 Lr

(18)

Cs =

1 1 · 2 . 2 δi ω0 Ls

(19)

As depicted by the flowchart in Fig. 4, the entire procedure of the proposed energy encryption scheme can be summarized in following steps. Step I: By arbitrarily setting the initial value of logistic map, a chaotic sequence can be generated by (14), which is utilized to

Flowchart of the proposed energy encryption scheme.

generate the security key by using (15) and (16) for the energy encryption. Step II: The working frequency of the primary side, namely the energy supplier, is continually regulated to vary around ω 0 in a chaotic way. At this stage, the frequency of the transferred energy is unknown to the secondary sides, namely the energy receptors. Meanwhile, the Cp is regulated according to (17), aiming to make the primary circuit work in the resonant state and thus reducing the unnecessary reactive power. Step III: The energy supplier works in the standby state until receiving a request from the receptor. First, the supplier verifies the corresponding identification. Then, the request from the unauthorized receptor is rejected and the supplier comes back to the standby state. For the authorized receptor, the request is accepted and the supplier delivers the security key. It should be noted that the rule of authorization can be revised according to various applications. In EV charging systems, for example, the unauthorized receptor may be the unpaid, unstable, or illegal vehicle. Step IV: After receiving the security key, the resonant and secondary capacitors are simultaneously regulated by using (18) and (19), respectively, aiming to achieve the maximum power transfer. Consequently, the encrypted energy can be decrypted by the synchronized regulation, which can effectively prevent unauthorized receptors from stealing the energy and thus significantly improve the security performance of the WPT system. IV. RESULTS To validate the proposed energy encryption scheme for MRCbased WPT systems, 3-D electromagnetic field analysis is performed by using JMAG. Then, the system simulation is carried out by using MATLAB/SIMULINK. In the exemplified prototype, the working frequency is chosen as 100 kHz. The key parameters of the three coils are listed in Table I. In addition, the corresponding experimentation is also conducted to verify the proposed scheme and simulation results, where the energy encryption algorithm is implemented by a digital-signalprocessing (DSP) microcontroller.


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TABLE I PARAMETERS OF THREE COILS Item Primary coil inductance (L p ) Primary coil internal resistance (r p ) Primary coil number of turns (N p ) Resonant coil inductance (L r ) Resonant coil internal resistance (r r ) Resonant coil number of turns (N r ) Secondary coil inductance (L s ) Secondary coil internal resistance (r s ) Secondary coil number of turns (N s ) Mutual inductance (L p r ) Mutual inductance (L r s )

Value 0.09589 mH 0.2215 Ω 20 0.09477 mH 0.07032 Ω 20 0.009372 mH 0.03565 Ω 10 0.005305 mH 0.007958 mH

Fig. 6. Electromagnetic field analysis under resonant coupling. (a) Magnetic flux line. (b) Magnetic flux density.

Fig. 5. Geometry of three coils. (a) Primary and resonant coils. (b) Secondary coil. (c) Displacements among coils.

A. Simulation Results Based on the topology of the MRC-based WPT system as depicted in Fig. 2, the setup mainly consists of the primary, resonant, and secondary coils. Fig. 5(a) depicts the dimensions of the primary and resonant coils, where both have a hollow circular shape with the inside diameter of 125 mm and the outside diameter of 215 mm. As depicted in Fig. 5(b), the secondary coil adopts the small-size design with the outside diameter of 90 mm and the inside diameter of 54 mm, aiming to facilitate testing the encryption performance at different positions with different magnetic flux densities. As shown in Fig. 5(c), the primary and resonant coils directly face each other, and the secondary coil has an offset of 40 mm from the resonant coil. In this MRCbased WPT system, the total transmission distance is 170 mm. Practically, the primary unit can be considered as the power supply mounted in a fixed place such as the EV charging panel in the parking lot or the roadway, while the resonant and secondary units can be both assembled in the energy receptor such

as the EV chassis. Additionally, the transmission distance can be changed based on various applications by adopting different coil designs. Figs. 6 and 7 depict the electromagnetic field distributions among the three coils when the WPT system works in the resonant and nonresonant coupling states, respectively. By comparing Figs. 6(a) and 7(a), it shows that the magnetic flux line of the resonant coupling state distributes more intensive and uniform than that of the nonresonant coupling state. In particular, the magnetic flux density of the resonant coil is remarkably increased by the resonant coupling magnetic field. Figs. 6(b) and 7(b) depict the magnetic flux density in the resonant coil under the two states. It shows that the maximum flux density can reach around 3.5 × 10−4 T in the resonant coupling state, while it is only around 5 × 10−5 T in the nonresonant coupling state. Thus, the magnetic field in the resonant coil can be purposely changed by regulating the resonant coupling state, hence controlling the energy received by the secondary coil. In this MRC-based WPT system, the capacitors of all three units need to be varied to fulfill the requirements of energy encryption and decryption. In order to realize the variation of capacitance, the capacitor array is utilized. As shown in Fig. 8(a), the capacitance of the primary, resonant, and secondary circuits can be regulated by switching on and off the capacitors. For exemplification, the WPT system is designed to work in eight resonant coupling states, which can be selected by using the logistic map as εi = floor(8ξi )

(20)

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TABLE II PARAMETERS OF CAPACITOR ARRAYS Circuit Primary Resonant Secondary

Capacitance (nF) Cp 1 4.7 Cr 1 4.7 Cs 1 1

Cp 2 10 Cr 2 10 Cs 2 2.2

Cp 3 22 Cr 3 22 Cs 3 4.7

Cs 4 47

Cs 5 100

Cs 6 220

Fig. 7. Electromagnetic field analysis under nonresonant coupling. (a) Magnetic flux line. (b) Magnetic flux density.

Fig. 9. Simulated waveforms at synchronized state. (a) Load voltage. (b) Load current. (c) Resonant circuit current.

Fig. 8.

Schematic diagram of the proposed energy encryption scheme.

where floor(·) is the rounding function, ξi is the chaotic Logistic sequence, and εi is the section selector. The parameters of three capacitor arrays are listed in Table II. Consequently, the capacitor arrays can be controlled according to the predefined switching table as depicted in Fig. 8(b). For example, when εi = 6, the switch can be controlled based on the value of the corresponding Section VI in the counter-clockwise direction. Then, the on–off states of the primary and resonant capacitor arrays are identical, namely 0(OFF), 1(ON), and 1(ON) in sequence, while the state of the secondary capacitor array is 0(OFF), 1(ON), 1(ON), 1(ON), 1(ON), and 1(ON). Meanwhile, the switching frequency of the power supply should be regulated to 91 kHz. In addition, when εi = 0, it means that no regulation is required, namely the switching state and frequency remain unchanged.


Fig. 10. Simulated waveforms of unsynchronized state. (a) Load voltage. (b) Load current. (c) Resonant circuit current.

Fig. 9 shows the simulated results for the authorized receptor, namely the MRC-based WPT system works in the synchronized state based on the security key. For the sake of easy implementation for experimentation, the load is set at around 20 W, which can readily be scaled up for various applications. From Fig. 9(a) and (b), it can be observed that the load voltage can reach around 10 V, while the load current can reach around 2 A in the synchronized state. When the MRC-based WPT system works in the unsynchronized state for the unauthorized receptor, as shown in Fig. 10, the load voltage is drastically reduced to around 0.25 V, while the load current is also reduced to around 0.05 A. The corresponding power received by the load is only 12.5 mW. Hence, it confirms that the unauthorized receptor cannot receive the wirelessly transferred energy. In addition, by comparing Figs. 9(c) and 10(c), it shows that the resonant coil current can reach a high level to equivalently turn on an energy transmission channel for the authorized receptor, and reduce to a very low level to turn off the energy channel for the unauthorized receptor, which matches with the previous electromagnetic field analysis results. Furthermore, Fig. 11 shows the performance waveforms of the proposed WPT system during state transition, where the receptor is authorized to receive the energy only within the time durations from 0.1 to 0.3 s and from 0.4 to 0.5 s. As depicted in Fig. 11(a) and (b), it illustrates that the energy can be wirelessly transferred to the receptor when the authorization sig-

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Fig. 11. Simulated waveforms during state transition. (a) Load voltage. (b) Load current. (c) Resonant current.

nal is activated. Also, the energy transmission channel can be effectively turned off when the authorization signal is inactivated. In addition, from the variation of resonant coil current as shown in Fig. 11(c), it can be observed that the resonant circuit current can be effectively controlled according to the authorization signal. Therefore, the proposed energy encryption scheme can significantly improve the security performance for MRCbased WPT systems. Besides, due to the capacitance is adjusted in a discrete way based on the capacitor array, while the inductances of those coils slightly vary with the frequency values, the transferred power inevitably has slight variations with respect to various frequency values. Thus, it can be observed that the output voltage and current waveforms in Fig. 11 exhibit some teethshape ripples over the duration of authorization. Nevertheless, these ripples would not significantly affect the power transfer capability. B. Experimental Results To experimentally demonstrate the proposed energy encryption scheme, a test bed is set up as shown in Fig. 12. The key parameters of the prototype are the same as that for simulation as listed in Tables I and II. The power source of the system is provided by a programmable ac power supply (Amplifier Research 75A250A). The transient currents are sensed by wideband current transducers (Tektronix TM502A). All measured waveforms are fed into a power analyzer (LeCroy WR6100A) for analysis and display.

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Fig. 12.


Prototype of the MRC-based WPT system.

Fig. 14. Experimental waveforms at unsynchronized state. (a) Load voltage (X: 1 s/div, Y: 2 V/div). (b) Load current (X: 1 s/div, Y: 0.5 A/div).

Fig. 13. Experimental waveforms at synchronized state. (a) Load voltage (X: 1 s/div, Y: 2 V/div). (b) Load current (X: 1 s/div, Y: 0.5 A/div).

By utilizing the acquired security key from the power supply, the resonant and secondary circuit can be simultaneously regulated to work in the resonant state. Fig. 13 shows the experimental results of the synchronized state for the authorized receptor, where the received power of the load can effectively reach about 20 W, namely the load voltage of 10 V and load current of 2 A. Additionally, the corresponding transmission efficiency is about 87%, where the main power loss is due to the conduction and switching losses of power switches and the internal resistance of coils. For the unauthorized receptor, the capacitor arrays of the resonant and secondary units cannot be appropriately regulated due to the absence of security key. Thus, the corresponding circuits work in the nonresonant state. Fig. 14 shows the experimental results of the unauthorized receptor, where the load voltage and load current are suppressed to an insignificant level. Moreover, Fig. 15 shows the current waveforms of the resonant coil. Technically speaking, Fig. 15(a) depicts the current of the resonant coil, which verifies that the energy transmission channel is effectively established for the authorized receptor in the proposed MRC-based WPT system. On the other hand, from the resonant current waveform as depicted in Fig. 15(b), it

Fig. 15. Experimental current waveforms of the resonant coil. (a) Synchronized state (X: 1 s/div, Y: 2 A/div). (b) Unsynchronized state (X: 1 s/div, Y: 2 A/div).

shows that the current of the resonant coil can be significantly suppressed without knowledge of the security key, which verifies that the energy transmission channel is effectively switched off for the unauthorized receptor. Besides, by changing the position of the secondary coil, the feasibility of the proposed energy encryption strategy at different magnetic flux densities can be also verified by both simulation and experimental results. Finally, it can be observed that the experimental results well agree with the theoretical analysis and the simulation results. It verifies that the proposed energy strategy can effectively transfer the energy to the authorized receptor, as well as to prevent the unauthorized receptor from stealing the energy.


C. Discussions and Recommendations This paper arouses attentions on the energy security issue for WPT systems, where the capacitor array method is utilized to improve the security performance of the wireless-transferred energy. However, it should be noted that the proposed energy encryption strategy still needs further improvement to optimize the transmission performance, enhance the encryption security, and enrich the control strategies in the system level. First, the exemplified capacitor array method can be further improved to enhance the encryption performance by properly increasing the section number, updating frequency, and the algorithm complexity of the security key. Meanwhile, the associated negative impact on the transmission performance should be also taken into account in future studies, such as the additional power loss, increased circuit complexity, and even dependence on the communication protocol. By systematically considering specific practical applications, it can achieve an optimal balance between the transmission performance and energy encryption. Second, the capacitance value can be adjusted in a continuous way instead of a discrete way, which can greatly increase the number of potential impedance values for the WPT circuit. For instance, the variable virtual capacitor technique can be utilized to realize the continuously adjustable capacitance to supersede the capacitor array [36], [37]. Thus, there will be no need to divide the capacitance value into sections so that the security key can be directly used to represent the desired capacitance. Consequently, the possibility of cracking the encryption strategy can be significantly reduced, therefore effectively improving the energy security for WPT systems. Third, the system-level control and management policy of energy encryption should be formulated in future studies, which is essential to standardize the energy encryption for WPT systems, especially for large-scale application scenarios. Based on this policy, there will be numerous kinds of applications for WPT with different levels of security such as the selective charging of electronic devices putting on the same charging pad or selective charging of EVs running on the same charging lane. V. CONCLUSION In this paper, a new energy encryption strategy has been proposed and implemented, which can significantly improve the security performance of wirelessly transferred energy. The MRCbased WPT system is adopted to illustrate the mechanism of the proposed secure energy transmission technology. The key is to chaotically regulate the switching frequency of the ac power in such a way that the energy transfer to those unauthorized receptors can be suppressed. Meanwhile, the authorized receptor can effectively receive the transferred energy by decrypting the energy with the acquired security key. Both the simulation and experimental results have validated that the proposed energy encryption strategy can effectively transfer the energy to the authorized receptor and meanwhile prevent the unauthorized one from stealing the energy; thus, significantly improving the security performance of WPT systems.

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Zhen Zhang (S’11–M’13) received the B.Eng. and M.Eng. degrees from Tianjin University, Tianjin, China, in 2004 and 2007, respectively, and the Ph.D. degree from The University of Hong Kong, Hong Kong, China, in 2014 He is currently an Associate Professor with the School of Electrical Engineering and Automation, Tianjin University, and a Research Scientist with the Department of Electrical and Electronic Engineering, The University of Hong Kong. His research interests include wireless power transfer techniques, electric drives, power electronics, and renewable energies.

K. T. Chau (M’89–SM’04–F’13) received the B.Sc. (Eng.), M.Phil., and Ph.D. degrees in electrical and electronic engineering from The University of Hong Kong, Hong Kong, China, in 1988, 1991, and 1993, respectively. Since 1995, he has been with The University of Hong Kong, where he is currently a Professor with the Department of Electrical and Electronic Engineering, and the Director of the International Research Center for Electric Vehicles. He is the Author of four books and more than 400 refereed technical papers. His research interests include electric and hybrid vehicles, power electronics and drives, and renewable energy. Dr. Chau is a Fellow of the Institution of Engineering and Technology and of the Hong Kong Institution of Engineers. He currently serves as a Coeditor of the Journal of Asian Electric Vehicles. He is a Chartered Engineer. He received the Changjiang Chair Professorship from the Ministry of Education, China, and the Environmental Excellence in Transportation Award for Education, Training, and Public Awareness from the Society for Automotive Engineers International.

Chun Qiu received the B.Eng. and M. Eng. degrees in electrical and electronic engineering from the Huazhong University of Science and Technology, Wuhan, China, in 2008 and 2012, respectively. He is currently working toward the Ph.D. degree at the Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China. His research interests include wireless power transfer, electric vehicles, and power electronics.

Chunhua Liu (S’05–M’10–SM’14) received the B.Eng., M.Eng., and Ph.D. degrees from the Department of Automatic Control, Beijing Institute of Technology, Beijing, China, and the Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China, in 2002, 2005, and 2009, respectively. He is currently serving as a Researcher with the Department of Electrical and Electronic Engineering, The University of Hong Kong. His research interests include energy conversion, integration, and distribution, including electric machines & drives, electric vehicles, renewable energies, wireless power transfer, vehicle-to-grid (V2G), and smart grid. He is currently focuses on wireless power transfer for smart energy conversion and distribution, V2G for smart energy distribution, permanent-magnet brushless machines for sustainable energy conversion, and renewable energies for integration to smart grid.